1. Field of the Invention
The present invention relates to a laser processing method of processing a workpiece by irradiation of a laser beam.
2. Description of the Background Art
Various technologies of processing a workpiece by irradiation of a pulsed laser beam (hereinafter, simply referred to as laser processing or laser processing technology as well) have been publicly known (for example, see Japanese Patent Application Laid-Open No. 2004-9139, International Publication WO 2006/062017, and Japanese Patent Application Laid-Open Nos. 2007-83309 and 2008-98465).
Japanese Patent Application Laid-Open No. 2004-9139 discloses the technique of forming, in dividing a die as a workpiece, a groove (break groove) having a V-shaped cross section along a planned division line by laser ablation to divide the die with this groove being an originating point. On the other hand, International Publication WO 2006/062017 discloses the technique of generating, in an irradiation region, a melting modified region (altered region) having an approximately V-shaped cross section in which a crystal state is more deformed compared with surroundings by irradiating a laser beam in a defocused state along a planned division line of a workpiece (object to be divided), to thereby divide the workpiece with a lowermost point of the melting modified region being an originating point.
In a case where a division originating point is formed using the technology disclosed in Japanese Patent Application Laid-Open No. 2004-9139 or International Publication WO 2006/062017, it is critical to form a V-shaped cross section (groove cross section or altered region cross section) having a constant shape along a planned division line direction that is a scanning direction of a laser beam for excellent division thereafter. To this end, for example, irradiation of a laser beam is controlled such that irradiation regions (beam spots) of a laser beam for each pulse overlap each other front and back.
For example, when R represents a repetition frequency (unit: kHz) and V represents a scanning speed (unit: min/sec), which are the most basic parameters in laser processing, a ratio V/R therebetween is an interval between centers of the beam spots. In the technologies disclosed in Japanese Patent Application Laid-Open No. 2004-9139 and International Publication WO 2006/062017, irradiation and scanning of laser beams are performed on the condition that V/R is equal to or smaller than 1 μm, so that beam spots overlap each other.
Further, Japanese Patent Application Laid-Open No. 2007-83309 discloses the mode in which a laser beam is irradiated with a light focus point directed to an inside of a substrate having a lamination part on a surface to form a modified region inside the substrate, and this modified region is used as an originating point for cutting.
Further, Japanese Patent Application Laid-Open No. 2008-98465 discloses a mode in which laser beam scanning is repeated a plurality of times on one separation line, to thereby form, vertically in a depth direction, a groove and a modified part that are continuous from each other in the separation line direction and internal modified parts that are not continuous from each other in the separation line direction.
On the other hand, Japanese Patent Application Laid-Open No. 2005-271563 discloses the processing technology using a laser beam of an ultra-short pulse having a pulse width such as psec order, in which minute melting trails consisting of a group of micro cracks are formed from a front layer portion to the surface of a workpiece (plate) by adjusting a light collecting point of a pulsed laser beam, to thereby form a linear easily separable region where such melting trails are continuously lining.
The technique of forming a division originating point by a laser beam and then performing division with a braker is superior to diamond scribing which is a conventionally-performed mechanically cutting method in that it is performed more automatically at higher speed, and with higher stability and accuracy.
However, in a case where a workpiece, in which a light emitting device structure such as an LED structure is formed on a substrate made of fragile and optically transparent material such as sapphire, is divided in units of chips (divided pieces), processing trails caused as a result of laser processing absorb the light generated inside the light emitting device, leading to a problem that light extraction efficiency from the device deteriorates. In particular, such a problem is serious in a case of a light emitting device structure in which a sapphire substrate having a high refractive index is used.
As a result of intensive studies, the inventor(s) of the present invention have found that the formation of minute asperity at a processed position of a workpiece with the use of cleavage properties or parting properties of the workpiece to reduce a total reflection index at the position solves the above-mentioned problem and is effective at achieving much higher light extraction efficiency compared with diamond scribing in which laser processing trails are not present, and that the asperity is preferably formed with the use of a laser beam of ultra-short pulse.
In Japanese Patent Application Laid-Open No. 2004-9139, International Publication WO 2006/062017, Japanese Patent Application Laid-Open Nos. 2007-83309, 2008-98465 and 2005-271563, such a problem is not mentioned and a mode in which cleavage properties or parting properties of a workpiece are used is not disclosed nor suggested.